Much activity is presently being directed towards the design and deployment of "point-to-multipoint" broadband access networks, wherein downstream signals are broadcast from a single head-end facility to multiple end user stations (i.e., via "point-to-multipoint" transmission), and upstream signals are transmitted from each respective end user to the head end facility (i.e., via "point-to-point" transmission). It presently anticipated that point-to-multipoint broadband access networks will be employed to support a variety of independent communication services, such as traditional two-way telecommunications, broadcast video (i.e., CATV) services and a full range of digital baseband services.
Given the wide variety of potential communication services to be supported over point-to-multipoint broadband access networks, it is desirable to provide efficient digital data transmission protocols for supporting both the downstream and upstream communication paths. Notably, such networks are well suited to support asynchronous transfer mode ("ATM") based data transmission, whereby data packets or "cells" are periodically assembled and transmitted from a sending node, and received and disassembled at a receiving node. In particular, ATM transmission enables the transmission of multiple services over a single communication path, whereby individual service bandwidth utilization may be optimized as a function of the statistical activity of each individual service.
By way of specific examples, a preferred system architecture and data transmission protocol for a point-to-multipoint broadband access network employing an ATM-based passive optical network ("PON") is disclosed and described in U.S. Application Ser. No. 08/826,633, filed Apr. 3, 1997, entitled "Data Transmission Over a Point-to-Multipoint Optical Network," now U.S. Pat. No. 5,926,478. A preferred system architecture and data transmission protocol for a point-to-multipoint broadband access network employing a shared coaxial medium is disclosed and described in U.S. application Ser. No. 08/772,088, filed Dec. 19, 1996, entitled "Network Architecture for Broadband Data Communication Over a Shared Medium," now U.S. Pat. No. 5,926,476. Both of these applications are assigned to the assignee of the present application and both are fully incorporated herein by reference.
The use of a relatively low noise, high speed point-to-multipoint optical network to support the two-way transmission of a wide variety broadband data services is especially desirable in that relatively large amounts of data may be transported in short time periods. However, in order to fully utilize the high speed, low noise environment provided by an optical network, the selected data transmission protocol(s) should be compatible with existing optical transmission standards, should most fully utilize the available carrier channel bandwidth, and should conform with applicable ATM transmission protocols.
In addition to the efficient and successful delivery of bearer information, such communication protocols must also account for the efficient and successful delivery of "system level" data, including what is commonly referred to as operations, administration and maintenance ("OAM") data. In particular, OAM functions are generally categorized into performance, fault, configuration, accounting, security and traffic management data.
For example, OAM functions needed in an ATM-based point-to-multipoint PON can be categorized by looking at the protocols chosen for data transfer, the physical equipment related functions, the functions required, and the services supported over the access network. A general categorization of the OAM functions, based on a "protocol stack" view of a PON system, is set forth as follows:
1. Physical layer: PA0 2. ATM layer: PA0 3. Media Access control ("MAC") functions: PA0 4. ATM Adaptation Layer ("AAL") level: PA0 5. Service Level: PA0 6. Hardware alarms:
(a) Optical layer, including transmit and receive power levels, ranging, synchronization, optical signal loopback, signal detection and frame alignment; and PA1 (b) PON transmission convergence, including cell slot and frame integrity, downstream frame and cell loopback, upstream cell loopback, error monitoring, cell rate de-coupling, cell delineation, performance monitoring, and fault localization. PA1 (a) ATM cell integrity, flow control, traffic congestion control, and service priority identification ("PTI") and processing; PA1 (b) Virtual path ("VP") alarm indication signals ("AIS"), remote defect indication ("RDI"), continuity check, forward performance monitoring, backward performance monitoring, loopback, system management functions, and fault and performance management; and PA1 (c) Virtual circuit ("VC") AIS, RDI, continuity check, forward performance monitoring, backward performance monitoring, loopback, system management functions, and fault and performance management. PA1 (a) Downstream bandwidth allocation and maintenance, upstream transmission bandwidth request processing, polling and sign-on of idle optical network units ("ONUs"); and PA1 (b) Upstream bandwidth requests based on traffic demand from each respective ONU, request grant identification and proper association with the upstream cell slot. PA1 (a) User data at this level is beyond the scope of the PON management; and PA1 (b) System signalling data is carried over AAL and is to be managed at the AAL level. PA1 (a) Equipment failures affecting the physical, MAC, and/or ATM layers; and PA1 (b) Internal failures, such as power or battery failures, backplane, board and component failures, and mechanical and environmental failures.
This is transparent to PON the interface and concerns only end-points and intermediate points that process each particular service type.
A generally accepted standard for OAM flows in broadband access networks is published in the ITU-T Recommendation 1.610 (draft published November 1995), entitled "B-ISDN Operation and Maintenance Principles and Functions," which is fully incorporated herein by reference. In accordance with the ITU-T 1.610 Recommendation, OAM functions in a broadband integrated services digital network (i.e., "B-ISDN OAM functions") are divided into the following stages: (1) performance monitoring, including the continuous or periodic monitoring of all network managed entities to verify their normal operation, resulting in maintenance event information; (2) defect and failure detection, including the detection of malfunction conditions through a continuous or periodic inspection, resulting in maintenance or alarm reports; (3) system protection, including minimizing of managed entity failure effects by blocking or replacing the entity--i.e., excluding the failed entity from further operation; (4) delivery of failure or performance impairment information to network management entities, e.g., such as alarm indication and status report delivery; and (5) fault localization, including internal or external testing to determine the impaired entity for fault localization.
In accordance with the ITU-T 1.610 Recommendation, B-ISDN OAM functions are essentially performed over five hierarchical "levels" associated with the ATM and physical layers of the protocol reference model, resulting in corresponding bidirectional "information flows" F1, F2, F3, F4 and F5, referred to herein as "OAM flows." Not all five hierarchical OAM flows F1-F5 must necessarily be employed in a particular network implementation--e.g., the OAM functions of a missing level may be performed at the next higher level. More particularly, the OAM flows F1-F5 constitute a means for in-band communication of information between peer OAM elements and functions of the broadband network, which collectively monitor the data path at different network levels. The transfer mode used for the information carried by these flows depends on the nature of both the particular level and the transport network.
By way of examples, for a physical layer based on the CCITT standard synchronous digital hierarchy ("SDH"), F1 to F3 flows are carried in synchronous channels in the overhead of the physical layer. For a cell-based physical layer, F1 to F3 flows are carried by physical layer OAM ( "PL-OAM") cells. For the ATM layer itself, the F4 flows are carried in cells distinguished by pre-assigned virtual circuit identifiers ("VCIs") in the virtual path, and the F5 flows are carried in cells distinguished by special PTI codes in the virtual circuit.
Existing industry standards addressing broadband OAM functionality are mainly directed to point-to-point backbone networks. Such networks, however, are fundamentally different from broadband access network architectures, and in particular, from point-to-multipoint networks. For example, while the data flows in a point-to-multipoint broadband access network are bidirectional, they are asymmetrical. As such, OAM flows in a point-to-multipoint broadband access network must be separately defined for "downstream" (i.e. from a head-end facility to the respective subtending terminals) and "upstream" (i.e., from the respective subtending terminals to the head-end facility) directions. Further, these OAM flows must be implemented through different mechanisms, carry different functions, and pertain to different end-points in each direction. In this context, the head-end and the subtending terminals are not peer entities, e.g., the terminals must report back fault conditions and performance degradation to the head-end, but the opposite is not applicable.
Thus, it would be desirable to define and provide specific OAM flow protocols for a broadband access network environment and, by way of example, show how such definitions and protocols would be applicable in a PON-based, point-to-multipoint broadband access network configuration.